1. Software that prescribes to, but does not provides any value or enhancement.
2. Software that provides nonsensical increasing or decreasing values to system resources.
3. Software that suggests, but does not deliver any improvements to lifestyle, productivity or success.
4. Software that makes the user happier once it has been run.

Below are examples of placeboware, but this is by no means an exhaustive list:
Memory Optimizers / Managers / Cleaners / Defragmenters
Flash Disk Optimizers / Defragmenters
Operating Systems that require a single mouse button